1. Technical Field
The present invention relates to a radio frequency power amplifier, and, in particular, to a technique for increasing the efficiency of a high-power radio frequency power amplifier.
2. Background Art
Radio frequency power amplifiers are devices which output an amplified input signal, and are widely used in mobile communication terminals and base stations. However, radio frequency power amplifiers require a large amount of power and consume most of the supplied power needed to drive radio frequency generator circuits or transmitter circuits, hindering the low power consumption of mobile communication terminals and base stations. Accordingly, there has long been a need for a radio frequency power amplifier with decreased driving power.
A radio frequency power amplifier has input and output characteristics such that when the input power is low, the input signal is amplified while maintaining constant gain (in the linear region). As the input power increases, nonlinearity of the amplifying element causes gain compression, and output power becomes constant regardless of the input power level (in the saturation region).
In general, radio frequency power amplifiers used in mobile communication terminals and base stations operate in the linear region and widely employ a Doherty amplifier to increase the power added efficiency (PAE) of the radio frequency power amplifier.
A Doherty amplifier includes a carrier amplifier and a peak amplifier connected in parallel. When the input power is low, only the carrier amplifier is driven; when the input power is high, both carrier and peak amplifiers are driven. As a result, the power added efficiency of the whole amplifier increases, regardless of the input power level.
Additionally, a class F amplifier, which is a generally known radio frequency power amplifier with the ability to operate with high efficiency, is an amplifier that reduces the amount of power consumed inside the amplifying element by tuning the termination condition of the harmonic wave and shaping the voltage waveform and current waveform applied to the amplifying element output terminal (For example see PTL 1).
FIG. 13 is a circuit diagram showing an example of a class F amplifier (class F amplifier circuit) that is a conventional radio frequency power amplifier using a field effect transistor (FET).
A conventional radio frequency power amplifier as shown in FIG. 13 includes: an amplifying element (FET) 804; a microstrip line 805 connected to output terminal A of FET 804, with a length corresponding to one-quarter of a wavelength λ (λ/4) of the fundamental wave of the input signal; a microstrip line (open stub) 806 and a microstrip line (open stub) 807, both with one end connected to an output terminal B of a microstrip line 805, and the other end open; an output matching circuit 808 for the fundamental wave of, the input signal; and a load resistor 809. The conventional radio frequency power amplifier shown in FIG. 13 also includes an input terminal 801 connected to a gate electrode of a FET 804 for inputting a radio frequency signal; and a choke inductor 803 for cutting a radio frequency signal, having one end connected to an output terminal A of a FET 804, and having the other end connected to a direct current power supply terminal 802 of a drain bias.
A harmonic wave control circuit including the microstrip line 805, the open stub 806, and the open stub 807 is included in the conventional radio frequency power amplifier shown in FIG. 13. Here, the microstrip line 805 has a line length of one-quarter wavelength (λ/4), the open stub 806 has a line length of one-eighth wavelength (λ/8), and the open stub 807 has a line length of one-twelfth wavelength (λ/12). Due to the open stub 806 having a line length of one-eighth wavelength, impedance at the point B is short circuited with respect to a second harmonic wave. Furthermore, due to the microstrip line 805 having a line length of one-quarter wavelength, load impedance as seen from the point A, which is the output terminal of the FET 804, is short circuited with respect to a second harmonic wave. Again, due to the open stub 807 having a line length of one-twelfth wavelength, impedance at the point B is short circuited with respect to a third harmonic wave. Furthermore, due to the microstrip line 805 having a line length of one-quarter wavelength, load impedance as seen from the point A, which is the output terminal of the FET 804, is opened with respect to a third harmonic wave. Thus, the conditions of the harmonic wave control circuit are satisfied. With the conditions satisfied, the drain voltage waveform becomes a waveform closer to a square wave, and the area of the overlapping portion of the drain voltage waveform and drain current waveform decreases. Consequently, the power consumed by the FET 804 decreases, resulting in extremely-high power added efficiency.
Another intended use of the radio frequency power amplifier is in home appliances that use microwaves, such as microwave ovens. When a radio frequency power amplifier is used, for example, in a microwave oven, it is preferable that the radio frequency power amplifier is used in a saturation region, not in a linear region, in order to operate with high efficiency and at high power. Compared to operation in a linear region, operation in a saturation region produces high power added efficiency, but signal distortion is also generated. For this reason, the radio frequency power amplifier is not suitable for use in the communications field, but it is suitable for use in the field of home appliances with microwaves, typified by the microwave oven. However, when use in a home appliance with microwaves is being considered, an output power at least one order of magnitude or greater than specified of a mobile communication base station is required.
In general, transistors that are compound semiconductors made with gallium arsenide (GaAs) have been widely used as an amplifying element for use in radio frequency power amplifiers. However, in recent years, new device architectures for high power operation, or devices made with new materials which are capable of high voltage operation, such as silicon carbide (SiC) or gallium nitride (GaN), have been developed more actively in various institutes.
In order to achieve high power performance of a radio frequency power amplifier, a transistor needs to operate at high current and high voltage. While operation under high current is possible by increasing transistor size, achievement of operation under high voltage is not easy, even by using SiC or GaN, both of which have a high dielectric breakdown field. A field plate structure is widely known to increase the breakdown voltage of a transistor.
FIG. 14A is a cross-sectional view of a conventional GaN-based field effect transistor (FET), while FIG. 14B is a cross-sectional view of a field effect transistor (FET) having a field plate structure.
As shown in FIG. 14A, a conventional GaN-based FET is formed on a substrate 700, on top of which is a buffer layer 701. On top of that, a GaN channel layer 702 and an AlGaN electron donor layer 703 made from GaN with added aluminum (Al) are formed with a heterojunction of the two layers. On top of the AlGaN electron donor layer 703, a source electrode 704, a gate electrode 705, and a drain electrode 706 are formed. Also provided between each of the source electrode 704, the gate electrode 705, and the drain electrode 706, is an interlayer film 707. Included above and between the electrodes is an interlayer film 708.
In the conventional FET shown in FIG. 14A, a two-dimensional electron gas (electrons) is generated in the interface between the GaN channel layer 702 and the AlGaN electron donor layer 703 by the heterojunction of the two layers. These electrons contribute to the current between the source electrode 704 and the drain electrode 706, and the current value can be controlled by the voltage value applied to the gate electrode 705.
Also, as shown in FIG. 14B of a FET having a field plate structure, a gate electrode 705A protrudes out over an interlayer film 707, stretching towards a drain electrode 706 in an eaves shape. Due to this configuration, the electric field across the drain electrode 706 and the gate electrode 705A gathered in the vicinity of the gate electrode 705A is relaxed, and a FET having a high breakdown voltage can be realized. Other conventional field effect transistors having a field plate structure include those with a source electrode that protrudes out to between a gate electrode and a drain electrode, and those with a second, additional source electrode placed in between a gate electrode and a drain electrode. Additionally, a FET having a field plate structure for both a gate electrode and a source electrode has also been proposed as a FET configuration to increase the breakdown voltage of a FET (PTL 2).
[Citation List]
[Patent Literature]
    [PTL 1] Japanese Unexamined Patent Application Publication No. 06-204764    [PTL 2] Japanese Unexamined Patent Application Publication No. 2008-277604